The eye is a highly specialized optical system. Its proper function requires tradeoffs between image quality at a single wavelength and the need to operate over a wide range of conditions., Retinal image quality is determined by both the refractive elements of the eye (cornea and lens) and the cone photoreceptors. The cone photoreceptors play a critical role, since they preferentially capture light from a particular location in properties of the pupil, and because they discretely sample the retinal image. The interaction of cones and the refractive properties of the eye is not well understood, and is highly variable across individuals. Understanding of this interaction has high clinical relevance, since over 1,000,000 patients a year are undergoing some form of refractive surgery. Refractive surgery does not yet take into account the large intra-subject variations. If future plans for enhanced refractive strategies are to improve vision then they must be designed with an understanding of the tradeoffs and optimizations involved in image formation. This proposal will make both basic measurements of all aspects of image formation, and ask questions as to whether the aberrations of the eye arise from random errors, or are actually in some sense beneficial, providing increased uniformity of image quality with changes in the spectral content of the retinal image. We will measure the optical quality of the eye over the visible spectrum using modern wavefront sensing techniques. Using modern optical techniques we will obtain measurements of cone directional sensitivity and cone packing density. We will validate new optical techniques using psychophysical measurements. We will combine information on the wave aberrations of the eye with the measurements of directional selectivity and packing density of the cone photoreceptors to obtain individualized estimate of retinal image quality for both monochromatic and polychromatic light. Retinal image quality will be examined as a function of age and refractive error. We will use these data to test whether the incorporation of cone information into the estimates of retinal image quality generates a better description of patient visual performance. Finally, we will test whether the cones can actively respond to changes in the eye by re-orienting. Our long-term goal is to understand the factors involved in the initial stages of image formation in the human eye, including the contributions of the cornea, lens, and cone photoreceptors, as well as their interactions.